1. Field of the Invention
The present invention relates to a soft X-ray generating apparatus and a soft X-ray lithography apparatus using this soft X-ray generating apparatus. More specifically, the present invention concerns a soft X-ray generating apparatus that can generate a large quantity of soft X-rays, and a soft X-ray lithography apparatus using this soft X-ray generating apparatus.
2. Discussion of the Related Art
Conventionally, in exposure equipment used in semiconductor manufacture, equipment with an exposure and transfer system has been widely used in which a photo-mask (hereafter referred to as a xe2x80x9cmaskxe2x80x9d) is irradiated by means of an irradiation optical system so that circuit patterns formed on the mask surface are projected and transferred onto a substrate such as a wafer, etc., via an image-focusing device. The substrate is coated with a resist, and the resist becomes photosensitive as a result of being exposed so that a resist pattern is obtained.
The resolving power w of an exposure apparatus is determined mainly by the exposure wavelength xcex and the numerical aperture NA of the image-focusing optical system, and is expressed by the following equation:
w=kxcex/NAxe2x80x83xe2x80x83(1)
k: constant
Meanwhile, the focal depth DF is determined by the following equation:
DF=xcex/2(NA)2xe2x80x83xe2x80x83(2)
As is clear from Equation (1), it is necessary to reduce the constant k, increase the numerical aperture NA, or reduce the wavelength xcex of the light source in order to reduce the dimensions of the minimum pattern that can be exposed.
K is a constant that is determined by the projection optical system and process, and ordinarily has a value of approximately 0.5 to 0.8. Methods for reducing this constant k are referred to as super-resolution in a broad sense. In the past, methods such as improvement of the projection optical system, deformed illumination and phase shift mask methods, etc., have been proposed; however, there are difficulties in terms of the patterns that can be used, so that the range of application of such methods is limited. Meanwhile, as is clear from Equation (1), the minimum pattern dimensions can be reduced if the numerical aperture, NA, is increased. At the same time, however, as is clear from Equation (2), this leads to the problem of a reduced focal depth. Accordingly, there are limits to the extent to which the NA value can be increased, and a value of approximately 0.5 to 0.6 is ordinarily appropriate.
Accordingly, the most effective method of reducing the minimum pattern dimensions is to shorten the wavelength xcex of the light used for exposure, and to simultaneously reduce the NA as well, since shortening the exposure wavelength alone will reduce the focal depth.
Currently, in the manufacture of semiconductor integrated circuits, a method is widely used in which an extremely fine pattern formed on a mask is projected in a reduced form and transferred onto a silicon wafer (coated with a resist) by means of visible light or ultraviolet light. However, as pattern sizes become finer, the diffraction limit is being approached even in the case of ultraviolet light, so that reduction and projection type exposure using soft X-rays with wavelengths even shorter than those of ultraviolet light, i. e., wavelengths of 13 nm or 11 nm, has been proposed.
In cases where soft X-rays with a wavelength of 13 nm or 11 nm are used, one conceivable candidate for the light source (soft X-ray source) used is a laser plasma X-ray source (hereafter referred to as xe2x80x9cLPXxe2x80x9d). When pulsed light emitted from a laser device is focused and directed onto a substance, if the irradiation intensity exceeds 1010 W/cm2, electrons are stripped from the atoms of the substance by the intense electric field so that the substance is converted into a plasma, and soft X-rays are radiated from this plasma. The brightness of the soft X-rays radiated from this plasma is extremely high, and a large quantity of soft X-rays can be obtained by generating the plasma at a high repetition rate. Furthermore, an LPX is extremely compact as an apparatus compared to synchrotron radiation facility, etc. Accordingly, LPXs show great promise, not only in the area of soft X-ray lithography, but also as radiation sources for X-ray microscopes and analysis devices, etc.
In cases where an LPX is used in soft X-ray lithography, the quantity of soft X-rays obtained from the light source is important. Since soft X-rays are strongly absorbed by all substances, ordinary lenses and reflective mirrors cannot be used. Accordingly, in order to obtain a high throughput in soft X-ray lithography, optical systems are constructed from reflective mirrors in which a multi-layer film is formed on the reflective surface. There is an intimate relationship between the combination of substances making up such a multi-layer film and the wavelengths reflected by the multi-layer film. In the case of an Mo/Si multi-layer film, a high reflectivity is obtained in the vicinity of a wavelength of 13 nm, while in the case of an Mo/Be multi-layer film, a high reflectivity is obtained in the vicinity of 11 nm. Accordingly, these wavelengths may be cited as candidates for the wavelengths used in soft X-ray lithography. However, even in the case of reflective surfaces formed using such multi-layer films, the limit of the reflectivity obtained for soft X-rays is approximately 70%. Assuming that approximately 10 reflective surfaces are used for reduction and projection type exposure, the transmissivity (reflectivity) of the optical system as a whole is extremely low, i. e., a few percent. Accordingly, in order to obtain a sufficient treatment speed (throughput) for a projection exposure apparatus, it is desirable that the quantity of soft X-rays generated by the light source be as large as possible.
In the case of an LPX using clusters formed by causing a gas to jet into a vacuum vessel as a target material, it is reported that the efficiency of conversion to a wavelength region of 13 nm or 11 nm (2.5% BW) is approximately 1 to 2%. The development of an LD-excited solid laser, which has an output of 1.5 kW as an exciting laser light source, has been promoted in order to obtain a sufficient throughput at this conversion efficiency. In the case of soft X-ray lithography, a method in which an annular band-form region is scanned is used in order to obtain a broad exposure area. When such scanning is performed, it is desirable that a continuous light source with a stable intensity be used in order to prevent the occurrence of irregularities in brightness within the exposure region. However, even in the case of an LPX, which is a pulsed light source, there is no problem if the repetition rate is on the order of kHz.
However, in order to increase the output of a conventional solid laser so that an output of 1 kW or greater is obtained, it is necessary to have a repetition rate on the order of kHz, and to increase the energy of one pulse to a high value. The development of such laser devices is currently being pursued; however, the development of a laser device which has such a large output, and which can operate stably over a long period of time, is not easy. Furthermore, even if such a laser device is developed, the resulting device would be extremely expensive.
Accordingly, there is a demand for a soft X-ray generating apparatus, which produces an output exceeding 1 kW, which is easy to manufacture, and which is equipped with an inexpensive pulsed laser light-generating device as an exciting pulsed light source, for use as an exciting light source that excites the laser plasma used in a soft X-ray lithography apparatus.
In cases where an LPX is used in a soft X-ray lithography apparatus, several optical systems for the purpose of projecting and exposing patterns of 0.1 xcexcm or less have been proposed. In these optical systems, only an out-of-axis circular-arc-form good-image region is utilized, and only a circular-arc-form region on the mask is projected. Accordingly, the overall pattern of the mask is treated by scanning the mask and the wafer. As a result, a high resolution can be obtained at a relatively high throughput. In order to utilize such an optical system, the circular-arc-form region on the mask must be efficiently illuminated. An illumination optical system of the type shown in FIG. 7 has been proposed as an illumination optical system that satisfies this condition (for example, see Japanese Patent Publication No. 11-312638.
In the proposed illumination system, illumination suitable for the above-mentioned lithography optical system is achieved by means of two fly-eye mirrors 2-a and 2-b. FIGS. 8(a) and 8(b) shows schematic diagrams of these fly-eye mirrors. As is shown in FIGS. 8(a) and 8(b), the first mirror 2-a is constructed from an aggregate of small mirrors that have a circular arc shape. The second mirror 2-b is constructed from an aggregate of small mirrors that have a rectangular shape that is close to square. The optical axes of these mirrors are matched as a result of the respective mirrors being eccentric.
The reason for constructing the illumination optical system as described above is that the light source cannot be viewed as a perfect point light source because the light source has a certain size that must be taken into account. By using such a construction, it is possible to achieve efficient illumination by means of soft X-rays.
In an LPX, the size of the light source is the same as the size of the plasma. If the region in which the material constituting the target is present is larger than the region irradiated by the exciting laser light, the size of the above-mentioned plasma is more or less equal to the size of the irradiated region. Then, the size of the plasma can be varied by changing the size of the irradiated region.
Furthermore, the wavelength distribution of the electromagnetic waves radiated from the plasma are closely connected with the temperature of the plasma that is produced. As the temperature of the plasma increases, shorter-wavelength electro magnetic waves (visible light, ultraviolet light, soft X-rays) are radiated. Accordingly, even in cases where a plasma is generated by means of pulsed light with the same pulse duration time and the same energy, the size of the plasma and the wavelength of the radiated soft X-rays will differ if the focusing diameter differs. Thus, a plasma generated by focusing pulsed laser light in a small region will be smaller than a plasma generated by focusing light in a large region, and will have a higher temperature. Consequently, the spectrum of the electromagnetic waves that are radiated will be shifted toward shorter wavelengths. For example, considering the case of black-body radiation, the peak of the radiation spectrum will be at a wavelength of approximately 13 nm when the temperature of the black body is approximately 30 eV. At higher temperatures, the peak will shift toward shorter wavelengths. In actuality, the soft X-rays radiated by the plasma depend on the electron structure of the material constituting the target, so that soft X-rays with an energy corresponding to the electron transitions are radiated. Consequently, soft X-rays corresponding to energy transitions that are close to the wavelength at which the radiation efficiency is high at the temperature in question are strongly radiated.
Currently, the outputs of laser light sources used for the excitation of a laser plasma are being discussed from the standpoint of the efficiency of conversion to soft X-rays in the principally utilized wavelength region. As an example, a soft X-ray output of 30 W may be considered necessary in order to cause the radiation of soft X-rays in the wavelength region utilized. In cases where this soft X-ray output is to be obtained from a laser plasma, the efficiency of conversion to soft X-rays radiated in a hemispherical solid-angular space (2 xcfx80sr) when a certain target is used is 1%. In a case where the soft X-rays in a region equal to xc2xd of this hemispherical solid angle (among the soft X-rays generated from the plasma) are to be input into the illumination optical system, it is considered that an output of 6 kW is required in the laser light source used for excitation.
However, in cases where a laser plasma is actually used as a light source in soft X-ray lithography, there are various limitations on the conditions of plasma generation. Accordingly, it is not always possible to discuss the problem in terms of conversion efficiency and output of the laser light source alone.
The first limitation is a limitation concerning the size of the plasma generated in a case where the illumination of the mask is taken into account. Since the solid angle through which light is input into the projection optical system is limited as seen from the mask, illuminating light from directions that exceed a solid angle of illumination matching the above-mentioned solid angle is wasted illuminating light. For this reason, it is required that the solid angle of illumination of the mask more or less coincides with the solid angle of input of the projection optical system.
In an optical system, the value of xe2x80x9csize of illuminated regionxc3x97illumination NAxe2x80x9d is conserved. Accordingly, in cases where the solid angle is stipulated, it becomes impossible in principle to use all of the light emitted from the light source for illumination if the light source exceeds a certain size. Consequently, in order to realize efficient illumination, it is desirable that the light source be limited to a certain size.
Furthermore, in the case of scanning exposure using an optical system of the type shown in FIG. 7, the illuminated region has a long, thin shape oriented in the direction perpendicular to the scanning direction. Accordingly, the permissible size of the light source differs according to direction, so that a light source that is small in the scanning direction is generally required.
In an optical system of the type shown in FIG. 7, if illumination is performed at a slit width of 1.5 mm and an illumination NA of 0.06, the light source may be viewed as being substantially a point light source as long as the diameter of the light source is smaller than about 50 xcexcm. As a result, the above-mentioned advantages of the fly-eye mirrors are obtained.
Secondly, there is a limitation imposed by the output of the exciting laser light source and the laser light irradiation intensity at which the maximum conversion efficiency is obtained. In order to obtain the utilized soft X-rays at a high conversion efficiency, it is desirable to generate a plasma which has a temperature at which the soft X-rays of that wavelength are efficiently radiated. For this purpose, irradiation of the target at the optimal irradiation intensity per unit area is necessary. Accordingly, in cases where the output per pulse of the exciting laser light has a certain energy, the irradiation area for achieving the optimal irradiation intensity is determined. In cases where this irradiation area is large, the area is substantially equal to the diameter of the plasma that is produced.
Because of these two limitations, the most efficient utilization of soft X-rays of the desired wavelength cannot always be achieved in cases where a single pulsed laser light source with a large output such as a conventional solid laser or excimer laser is used as the laser light source that excites the plasma used as a soft X-ray source in a soft X-ray lithography apparatus.
When a laser plasma is generated, the laser light source used for excitation is oscillated at the maximum output in order to maximize the quantity of soft X-rays obtained. When this maximum output is obtained, the pulsed energy and repetition rate are more or less fixed at certain values in a solid laser or excimer laser. As a result, under conditions other than these fixed values, the time-averaged output of the laser drops. Since the energy per pulse is regulated under the conditions that produce this maximum output, the irradiation area cannot be freely selected even in cases where it is desired to perform irradiation at an irradiation intensity per unit area that is suitable for generating a plasma of the desired temperature. Since the irradiation area more or less coincides with the size of the plasma, the size of the light source cannot be freely selected. In cases where the size of the plasma exceeds the size of the light source suitable for illumination, either the irradiation intensity must be increased while sacrificing optimization of the conversion efficiency, or else the irradiation area must be increased while sacrificing efficient utilization of the generated soft X-rays in illumination.
Furthermore, in order to optimize the efficiency of conversion to soft X-rays in the desired wavelength region, it is necessary to effect a relative decrease in the quantity of X-rays of other wavelengths that are radiated. These soft X-rays are absorbed by a reflective mirror, etc., and converted into heat, thus causing thermal deformation of the reflective mirror. There is a danger that such deformation of the reflective mirror will cause a drop in the treatment speed of the apparatus as a whole; accordingly, it is desirable that soft X-rays of a wavelength not utilized be minimized.
An object of the present invention is to provide a soft X-ray generating apparatus that obtains a large quantity of soft X-rays, and a soft X-ray lithography apparatus using this soft X-ray generating apparatus.
Another object of the present invention is to obtain an X-ray generating apparatus which can efficiently generate soft X-rays of a desired wavelength.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a soft X-ray generating apparatus comprises a vessel, a target within the vessel, a fiber optic light source for irradiating the target within the vessel, the fiber optic light source providing a plurality of exciting energy beams for irradiating the target to generate a plasma for radiating soft X-rays, the fiber optic light source for irradiating substantially the same position of the target with each of the exciting energy beams.
In a further aspect, the present invention contemplates a soft X-ray generating apparatus comprising a vessel, a target within the vessel, and a plurality of pulsed laser light sources for irradiating the target within the vessel, the pulsed laser light sources providing a plurality of exciting energy beams for irradiating the target to generate a plasma for radiating soft X-rays, each of the laser light sources being controllable to cause the plasma generated as a result of irradiation of the target to have an electron temperature in the range of approximately 20 eV to approximately 100 eV.
The present invention further encompasses a method for generating soft X-rays comprising the steps of providing a target within a vacuum vessel, providing a plurality of sources of laser light, and irradiating the target with laser light from the sources to generate a plasma that radiates soft X-rays, the step of irradiating including the step of individually controlling the laser light sources to set the temperature of the plasma in the range of approximately 20 eV to approximately 100 eV.